Prior traffic monitoring systems have typically involved a rubber tube, referred to as a "road tube", having one closed end and the other end in communication with a pressure transducer. The transducer outputs a signal, normally a voltage, responsive to variation of the air pressure in the tube. Accordingly, when a vehicle tire passes over the road tube, momentarily flattening a portion of it, the transducer outputs a signal exhibiting a peak responsive to the changes in pressure in the tube.
Several problems are well known to exist with conventional traffic sensing devices of this type. One is that vehicles passing over the tube at high speeds near the transducer will naturally produce a much more pronounced pressure wave in the tube than vehicles passing at lower speeds or further away from the transducer. Heavier vehicles similarly cause greater signal excursions than do light vehicles. As explained in detail in U.S. Pat. No. 3,949,355 to Newmeyer, these differences in the signal amplitude can be so pronounced that the pressure differential caused by passage of a heavy, fast vehicle overloads a transducer optimized to detect smaller signal variations, while low amplitude signals may not be detected by a transducer capable of reliable detection of high amplitude signals. According to the Newmeyer patent, the transducer assembly may be provided with a venting port to limit high amplitude excursions in the pressure, thus allowing a single transducer to be more appropriately matched to the amplitude of waves received in the road tube.
The Newmeyer patent also recognizes that a particular tire rolling over the road tube may generate a plurality of peaks in the pressure signal, e.g., due to reflection of the initial pressure peak from the ends of the tube. The Newmeyer patent attempts to solve this problem by disabling the traffic counting device from counting any additional vehicles after a particular pulse has been detected. A varying time delay is provided which requires the operator to set up the system in response to anticipated traffic conditions. Departures of the actual traffic from the anticipated traffic--either in traffic density, traffic rate or vehicle type--may cause the system to miscount the actual vehicles.
The Newmeyer patent also recognizes that certain spurious reflections occurring after the primary pressure pulse are normally of steadily decreasing amplitude due to attenuation of the pressure wave in the resilient tube. Accordingly, the Newmeyer patent suggests at column 7, lines 15-25 that a pulse amplitude threshold, to which incoming pulses are compared to determine whether they are likely to have occurred in response to a countable event, should be decreased over time. However, this solution is complicated and can lead to further inaccuracies.
Newmeyer also notes that a single set of vehicle wheels may cause a initial pulse in pressure of a first amplitude and may cause a second pulse of higher amplitude, e.g., due to whipping of the tube against the road, or if a vehicle crosses the tube at an angle such that its wheels do not compress the tube simultaneously. In order to remove these effects from contributing spurious counts to the traffic measurement, Newmeyer again suggests disabling the system from counting a second pulse within an operator-chosen predetermined period after a first pulse. However, as noted above, this approach demands that the operator set the system up correctly for the anticipated traffic conditions. This may or may not always be possible or convenient.
Accordingly, it can be seen that there exists a need in the art for an improved traffic monitoring system of increased sophistication over those available in the art, which provides accurate discrimination between fast and slow vehicles, and between heavy and light vehicles, which refuses to respond to spurious reflection of a pressure wave within the tube, and which is capable of distinguishing closely spaced axles such as those of two-axle trailers from one another. Simply providing an operator-selectable time delay together with means for attenuating higher amplitude pulses, as disclosed in the Newmeyer patent, is not sufficient. Ideally a device which unambiguously detects passage of a vehicle over the road tube is needed.